When sagging cable between structures, it is critical that the installation conform to engineered sag charts. Thermal contraction, combined with icing and wind, can pull down towers or poles if the cable is not sagged properly. Current methods of checking sag are: (1) optical survey; (2) visual approximation; (3) dynamometer; and (4) return wave timing.
While each of these techniques are used under various circumstances, they are all subject to limitations. The optical survey technique is labor intensive, the visual and return wave timing are subject to inaccuracies, primarily due to operator inexperience and human error, and the dynamometer has gained limited acceptance in the field.
The principal of measuring sag or tension of a conductor by means of wave timing has long been recognized. A mechanical wave initiated near one support will travel to the next support, reflect back, and pass back and forth repeatedly between supports. The sag of the conductor can be determined by timing the wave returns in seconds, and converting the measured value of time to sag with the following equation: ##EQU1## where sag is measured in inches, t is the time in seconds for a generated wave to return to the point of origin, and n is the number of return waves counted. The tension in a conductor is proportional to the velocity of a mechanical wave propagating along the conductor as given in the following equation: ##EQU2## where T is line tension measured in lbs, V is mechanical wave velocity (ft/sec), .mu. is the unit weight of the conductor (lb.sub.m /ft), and g.sub.c is the gravitational constant (32.2 lb.sub.m -ft/lb.sub.f -sec.sup.2). The fundamental mechanics equation for calculating conductor sag is: ##EQU3## where L is the span in feet, and T.sub.O is the conductor tension at midspan (lb.sub.f). By combining equations [2] and [3] and allowing for only one return wave, the following equation is derived, identical to equation [1], except for the units for sag: EQU Sag.sub.ft =1.0063t.sup.2 [ 4]
While it is possible to calculate the sag, or tension if the span is known, by measuring the time taken by a mechanical wave to propagate down the full span and back, this approach has several disadvantages. One disadvantage is the level of impact required for a measurable wave to propagate along larger conductors. This is a well-known problem with the return wave method on large conductors with long spans and one of the problems addressed by this invention. Another disadvantage is the unknown effects that conductor attachment/end-fixity may have on the propagating wave, end fixity referring to the wave dampening and phase shifts in the wave resulting in the transducers not being able to detect the return wave accurately or reliably.
From the brief discussion of the prior art, it can be seen that there is still a need for a reliable and accurate method and device to calculate cable sag with a minimum of operator intervention.